The Third (3rd) Generation Partnership Project (3GPP) Long Term Evolution (LTE) is an evolving high speed, high capacity standard for cellular communication. Currently, UE equipped with LTE technology utilize LTE Packet Switched (PS) domain to handle data traffic while the voice traffic is handled by legacy Circuit Switched (CS) networks (legacy Radio Access Technologies (RATs)) such as Global System for Mobile communications (GSM), Universal Mobile Terrestrial System (UMTS) and GSM EDGE Radio Access Network (GERAN). To support CS domain communication such as voice calls, the UE in LTE has to switch to CS supporting legacy RATs.
The Circuit Switched Fallback (CSFB) procedure standardized by 3GPP technical specification enables the UEs to perform this switching during Mobile Originated (MO) or Mobile Terminated (MT) calls. The 3GPP standard defines Inter-RAT (I-RAT) for mobility (switching) between LTE and earlier 3GPP technologies such as UMTS and GERAN. To acquire a legacy RAT during I-RAT switching, the 3GPP standard provides redirection based CSFB procedure.
In accordance with existing 3GPP standard for redirection based CSFB procedure, when the CS call is attempted while the UE is in LTE, then the UE starts an Extended Service Request (ESR) procedure. When the ESR is received by Evolved Universal Terrestrial Radio Access Network (EUTRAN) Node B (eNB) of the LTE network, the eNB provides the UE with Radio Resource Control (RRC) connection release along with redirection request including the target RAT frequency on to which the UE can camp for CS call.
If a user attempts to make normal or an emergency call, when the UE is camped on EUTRAN network, the CSFB Call Request may fail because of various reasons. The reasons could be temporary causes like Mobile Switching Center (MSC) not reachable, network congestion, temporary radio related problems (for example, random access failure, Radio Resource Control (RRC) connection failure) or permanent causes (CS domain not available) and so on.
Further, in case of temporary or radio failure causes according to specification (3GPP 24.301 TS), it is mentioned that the UE should move to GERAN or UTRAN. However, there is no information about RAT, to which UE has to select in case of a voice call. The lack of this information may reject or delay the call till the registration is completed on GERAN or UTRAN. Hence, it is important for the UE to maintain RAT Info which will help to camp on to the cell immediately and process the CS call without long delay.